Mass spectrometry (MS) is a powerful analytical technique that is used for the qualitative and quantitative identification of organic molecules, peptides, proteins and nucleic acids. MS offers speed, accuracy and high sensitivity. The development of ionisation techniques and mass analysers over the last decade has enables MS to solve a wide variety of problems. The introduction of Electrospray ionisation (ESI) greatly expanded the role of MS in pharmaceutical analysis. One of the characteristic features of ESI is the generation of multiply charged ions for large molecular weight compounds (e.g. proteins, peptides). These differently charged molecules enable accurate determination of the molecular weight of these compounds and their analysis in complex biological media.
In ESI, the analyte solution is typically introduced into a capillary which is electrically conductive or has a conductive coating. An electric potential is applied between the capillary and a counter-electrode. The analyte solution extends from the tip of the capillary in a shape known as the Taylor cone. The applied potential accelerates charged droplets from this cone towards the counter-electrode. The droplets reduce by fragmentation or evaporation to individual ions, and these are accelerated, typically through an aperture in the counter-electrode, into the mass analyser. Important features of ESI are the simplicity of its source design, and its capability to operate with solutions at atmospheric pressure. This means ESI may be coupled to high performance liquid chromatography (HPLC) for analysis of complex mixtures. The HPLC/MS combination uses the separation of HPLC with the detection of MS. ESI is also extremely sensitive. Furthermore, ESI is a soft ionisation technique that yields a simple, unfragmented and easily interpreted mass spectrum in which molecules typically correspond to the base peak. ESI is the method of choice of the characterisation of drug-bearing compounds and can be applied to over 90% of organic compounds in pharmaceutical research.
In the field of compound analysis it is known to use multiplexed, or MUX, systems with for example 4 to 8 channels feeding into a single mass analyser. However ‘cross-talk’ between the tips is a problem which can result in cross-contamination of sample sprays, thereby limiting the expansion of these systems to high numbers of channels. A further problem arises in the possibility that ions from previous stream are often still present. Furthermore when providing a plurality of channels, a separate bank of binary pump, splitters, LC and UV detector is required for each channel. If the cost and size of the ESI-MS system could be reduced, users could opt for arrays of ESI-MS systems running in parallel with maximum throughput and zero cross-talk.
HPLC flow splitters are often used to couple mass spectrometers to liquid chromatographs to reduce the amount and concentration of sample delivered to the mass spectrometers. This is particularly useful in automated systems to avoid unwanted MS inlet overload. Splitting is also required for applications in which a second detector or fraction collection device is used parallel to the MS (e.g. UV detector). HPLC/MS flow splitting is typical in the automated analysis of combinatorial libraries, drug metabolites and the characterisation of impurities.
In traditional HPLC/MS systems, the use of a postcolumn splitter decouples the chromatographic and Electrospray flow rates. The column operates at a high flow rate to provide optimal resolution, while the ESI source operates at a lower flow that is compatible with Electrospray or pneumatically assisted Electrospray. However, the integration of the Electrospray electrode with the column narrows the flow range that can be used. Thus, it becomes desirable to use electrodes with as broad a flow as possible.
For HPLC/MS with a low flow rate (100-200 μL/min), the sample solution can be sprayed directly into the ESI source. However, most samples in the pharmaceutical industry require HPLC separations at high flow rates (0.5-2 mL/min). A postcolumn split is often used to reduce actual flow rates to the ESI source to 40-200 μL/min. HPLC columns with smaller diameters are used for low concentrations of organic compounds and biomolecules and have flow rates of 1-40 μL/min. Alternatively, a nanoflow device (e.g. capillary LC) can deliver a sample solution directly to a nanospray source for analysis.
High flow rates are important to ensure compatibility with most HPLC systems. To initiate a spray requires very well defined electric fields; therefore factors such as applied voltage, needle diameter and position are critical. However, because electrospray is relatively difficult to achieve and maintain for traditional high flow rate ESI sources, pneumatic, ultrasonic or thermal nebulisation is also required to break up droplets in a process called desolvation. Such desolvation techniques add greatly to source complexity and cost.
Operating electrospray at high flow rates is forcing the process into an unnatural state, where stabilisation of what is called the Taylor Cone and formation of aerosol droplets are practically impossible with electric fields alone. To generate stable ion currents one must provide additional energy input, in the form of pneumatic nebulisation and heat, to force droplet formation, leaving the task of droplet charging to the electric field. Proper implementation of this additional energy is of overriding concern in the design of high flow rate systems, far overshadowing in importance other details of the Electrospray process such as Taylor Cone formation and stabilisation. For nanoflow techniques the opposite is true; factors affecting the formation and stabilisation of the Taylor Cone are of paramount concern. Other forms of external energy input to generate charged droplets are not required because the electric field is sufficient to charge the liquid and simultaneously generate an aerosol.
Nanospray sources operate in the low microliter per minute flow ranges. Nanospray involves using a low flow rate and a small needle diameter. The spray is introduced directly into the vacuum interface without pneumatic, ultrasonic or thermal nebulisation, reducing system cost and complexity. Nanospray permits the use of low flow techniques like microcapillary liquid chromatography (μLC) and capillary electrophoresis. Very small samples can be separated quickly and efficiently and analysed over a long period of time. Another benefit arises from the reduction in onset potential that comes with decreasing the needle diameter. This facilitates the use of aqueous solutions and reduces the likelihood of corona discharge.
The essence of the nanoflow method is to reduce the flow rate of the sprayed sample liquid by orders of magnitude below the microliter per minute regime. As stable flows are achieved at lower and lower flow rates, the efficiency of the ionisation process improves approximately in proportion to the flow rate reduction. Even though the sample molecules enter the sprayer at a much lower rate than with the high flow systems, the signal per unit time detected by the MS remains constant and can often be seen to improve by factors of 2-3.
For a given mass of sample injected, the analyte concentration, [A] is inversely proportional to the square of the column internal diameter, d. As the column diameter is reduced, the optimum flow rate Q also lowers by the same function.
Similarly, the ionisation efficiency E increases with lower flows.[A]∝1/d2; Q∝1/d2; E∝1/d2  Equation. 1
The outer diameter of the tip at the end of the capillary electrode establishes the minimum voltage required to produce sufficient electric field strength to initiate the Electrospray process. As such, sharper tips can generally be operated closer to the entrance aperture of the mass spectrometer. The taper of the channel leading up to the exit aperture and the restriction to flow it imposes also have an effect; long narrow channel results in flows somewhat lower than expected for a particular diameter.
At a lower cone voltage, the multiply charged ions are present at high relative abundances. For example, doubly charged ions of small peptides are intrinsically less stable than their singly charged analogs, and they can easily fragment to form singly charges ions. Low cone voltages can therefore be used to generate multiply charged ions of large molecules, permitting their detection by instruments with limited mass to charge range.
Because the spray is generated by strictly electrostatic means, the needle diameter, position and applied potential are critical. The potential Von (kV) required for the onset of electrospray is related to the radius r (μm) of the electrospray needle, the surface tension of the solvent, γ (N/m), and the distance d (mm), between the needle tip and the counter electrode, which is sometimes also the vacuum orifice:Von≈0.2√(rγ)ln(4000d/r)  Equation. 2
With methanol as the solvent (γ=0.0226 N/m), a spray needle radius of 50 μm, and a needle-counter electrode distance of 5 mm, the onset potential is 1.27 kV. Changing the solvent to water (γ=0.073 N/m) increases the onset potential to 2.29 kV. A possible problem with high applied potentials is that they can cause electric discharge from the capillary tip.
One solution to the problem of electric discharge is to reduce the needle diameter. In the pure water example changing the needle diameter from 50 μm to 10 μm decreases the onset potential from 2.29 kV to 1.3 kV. A reduction in the potential required to initiate a spray is one of several benefits of nanospray techniques.
Another solution is to reduce the needle-counter electrode distance. For example, for a spray needle radius of 50 μm, reducing the needle-counter electrode distance from 5 mm to 100 μm decreases the onset potential from 1.27 kV to 442 V.
Both these solutions require accurate alignment of the needle. Today, in order to achieve the necessary alignment, nanospray capillaries are mounted on an assembly of carefully machined stainless steel and ceramic parts, and located using expensive micro-positioners typically costing tens of thousands of dollars. A video camera is often included to help the user find the optimum position for Taylor cone formation, adding yet more cost.
There is therefore a need to provide a device and method that can provide for integration and alignment of the necessary components for such analytical instruments.